Semiconductor device and electronic device including the same

ABSTRACT

A semiconductor device capable of inputting signals and power without the use of an FPC is provided. The semiconductor device includes a first substrate and a second substrate. A receiver antenna is provided on a surface side of the first substrate. The second substrate is provided with a transmitter antenna and an integrated circuit. The second substrate is attached on a back side of the first substrate. The receiver antenna and the transmitter antenna overlap with each other with the first substrate provided therebetween. Thus, the distance between the antennas can be kept constant, so that signals and power can be received highly efficiently.

TECHNICAL FIELD

The present invention relates to semiconductor devices and electronicdevices including the semiconductor devices. In particular, the presentinvention relates to displays and electronic devices including thedisplays.

BACKGROUND ART

In recent years, flat panel displays have been widely used. Flat paneldisplays are used in a variety of devices such as liquid crystaltelevisions, personal computers, cellular phones, digital cameras,personal digital assistants, and portable audio equipment. Displaysincluding liquid crystals, OLEDs, electrophoretic elements, and the likeare widely used. An active-matrix display where transistors are arrangedin matrix in a pixel portion is presently pervasive.

FIG. 10 is the external view of a substrate used in a conventionaldisplay. A substrate 901 used in the conventional display includes apixel portion 902, a signal line driver circuit (also referred to as asource driver) 904, and a scan line driver circuit (also referred to asa gate driver) 903. In addition, signals and power needed in the scanline driver circuit 903 and the signal line driver circuit 904 are inputfrom the outside through a flexible printed circuit (FPC) 905 (forexample, see Reference 1). Here, for the scan line driver circuit 903and the signal line driver circuit 904, transistors provided over thesubstrate that are similar to transistors in the pixel portion may beused, or an IC chip may be attached to the substrate by chip on glass(COG).

REFERENCE

-   [Reference 1] Japanese Published Patent Application No. 2008-233727

DISCLOSURE OF INVENTION

Connection with the use of the FPC is as follows. The FPC 905 and awiring provided over a glass substrate or a flexible substrate such as aplastic substrate are bonded to each other with a conductive resin. Thesize of a plurality of terminals of the FPC is about 100 μm×1 mm each,and the contact area is not very large. Thus, the connection strength ofa portion where the wiring provided over the substrate and an FPCterminal are bonded to each other is not very high, so thatdisconnection might occur when vibration is caused or the temperature ischanged. In particular, when a flexible substrate is used, the substrateis bent; thus, the wiring and the FPC terminal are disconnected to eachother due to vibration, and contact failure might occur. In that case,signals and power needed in the display do not spread, which leads tothe malfunction of the display in some cases. In particular, the numberof signal lines is large; thus, the signal lines have high possibilityof malfunctions in connection portions. Therefore, a method forinputting signals and power to the display without the use of an FPC hasbeen demanded.

In view of the foregoing problems, it is an object to provide a noveldisplay capable of inputting signals and power without the use of anFPC. Without limitation to the display, it is an object to provide anovel semiconductor device capable of inputting signals and powerwithout the use of an FPC.

One embodiment of the present invention is a semiconductor devicecapable of inputting signals and power wirelessly without the use of anFPC. Specifically, the semiconductor device includes a first substrateand a second substrate. A first signal antenna and a first power antennaare provided on a surface side of the first substrate. The secondsubstrate is provided with a second signal antenna and a second powerantenna. The second substrate is attached on a back side of the firstsubstrate. The first signal antenna and the second signal antennaoverlap with each other with the first substrate provided therebetweento be fixed to each other. The first power antenna and the second powerantenna overlap with each other with the first substrate providedtherebetween to be fixed to each other. The first signal antenna and thefirst power antenna are receiver antennas. The second signal antenna andthe second power antenna are transmitter antennas.

In the semiconductor device, the first signal antenna and the firstpower antenna are provided separately. However, without separateprovision of the antennas, one antenna can serve as a signal antenna anda power antenna. In that case, one first antenna which serves as asignal antenna and a power antenna is provided over the first substrate,and one second antenna which serves as a signal antenna and a powerantenna is provided over the second substrate. The structures of theother components can be similar to those described above. In otherwords, the second substrate is attached on the back side of the firstsubstrate, and the first antenna and the second antenna overlap witheach other with the first substrate provided therebetween to be fixed toeach other. The first antenna is a receiver antenna, and the secondantenna is a transmitter antenna.

The number of signal lines is large; thus, in the case where the signallines are directly connected, the number of connection portions islarge. Thus, at least in a signal processing portion, signals aretransmitted and received wirelessly (without contact). Accordingly, theproblem of bad connection in an FPC terminal portion can be solved.

Further, the number of power lines is small. For example, the number ofpower lines per substrate can be two. Since the number of power lines issmall in this manner, in a power source portion, a wiring provided overthe first substrate can be directly connected to a wiring provided overthe second substrate or a different substrate. In that case, the powerlines can be connected to an external terminal with the use of an FPC orthe like. Accordingly, it is possible not to provide the first powerantenna and the second power antenna in the semiconductor device.

In the semiconductor device, plural first signal receiver antennas andthe plural second signal transmitter antennas can be provided. Whenplural sets of signal antennas (signal receiver antennas and signaltransmitter antennas) are provided in this manner, the transmitting andreceiving speed of signals can be improved. En one embodiment of thepresent invention, the problem of bad connection in a portion connectedto the outside is less likely to be caused as compared to the case wherean FPC or the like is used. Accordingly, a structure where the number ofsignal input/output portions (i.e., signal receiver antennas and signaltransmitter antennas) is increased can be easily employed.

In the semiconductor device, the first substrate and the secondsubstrate can be attached to each other with an adhesive or the like. Inaddition, a material into which an insulating filler is mixed may beused as an adhesive. When a material into which an insulating filler ismixed is used as the adhesive, the thickness of the attachment portion(i.e., bond portion) can be made more uniform.

A region where the first substrate and the second substrate are attachedto each other can be a region including a region where the antennas areprovided over the first substrate and the second substrate. Therefore,the area of the region where the first substrate and the secondsubstrate are attached to each other can be the same as or larger thanthe area of the antennas. The antennas have a certain size. Thus, theattachment portion has a certain size. Accordingly, the bond strength ofthe attachment portion can be made high.

In the semiconductor device, a flexible substrate can be used as thefirst substrate. Even when the substrate (the flexible substrate) isbent, in one embodiment of the present invention, the distance betweenthe antennas can be kept constant, so that signals and power can bereceived highly efficiently. In this manner, a semiconductor deviceaccording to one embodiment of the present invention can have variousstructures when the substrate is bent. Any substrate can be used as longas it has a certain thickness and includes a material which transmits anelectromagnetic wave with frequency used for transmission and receptionof signals and power. An insulating material can be used as a materialwhich transmits an electromagnetic wave with frequency used fortransmission and reception of signals and power. In addition, a flexiblesubstrate can be used as the second substrate attached to the firstsubstrate. When a flexible substrate is used as the second substrate, inthe case where the first substrate is bent, the second substrate is bentsimilarly. Therefore, even in the case where the substrate is bent, thedistance between the antennas can be kept constant.

In the semiconductor device, a thin-plate or film-like substrate with athickness of 0.1 to 3.0 min can be used as the first substrate. Here, athin flexible substrate is referred to as a film-like substrate. When athin-plate or film-like substrate is used, signals and power can bereceived highly efficiently. Further, a phenomenon that communicationfailure occurs only when the transmitter antenna and the receiverantenna are placed in a certain distance, i.e., a dropout phenomenon canbe prevented. This is because the distance between the two antennasprovided on the surface side and the back side of the first substrate isdetermined mainly by the thickness of the first substrate. In the casewhere the thickness of the first substrate exceeds the above range, thedropout phenomenon might occur. However, the thickness of the firstsubstrate is not necessarily limited to the above range. The firstsubstrate can have any thickness as long as signals and power can betransmitted and received and the dropout phenomenon does not occur.

In the semiconductor device, the first substrate can have a pixelportion including a plurality of pixels. Each of the plurality of pixelscan have a transistor and a display element.

In the semiconductor device, the first substrate can have a pixelportion including a plurality of pixels, a scan line driver circuit, anda signal line driver circuit. Each of the plurality of pixels can have atransistor whose on/off is controlled by the scan line driver circuit(such a transistor is also referred to as a switching transistor) and adisplay element to which an image signal is input from the signal linedriver circuit through the transistor.

In the semiconductor device, a liquid crystal element, a light-emittingelement, or an electrophoretic element can be used as the displayelement. When an electrophoretic element is used, power consumption canbe reduced. Alternatively, in the case where a liquid crystal element isused, a reflective liquid crystal element is preferably used. Areflective liquid crystal element can be obtained by formation of areflective electrode as a pixel electrode. Thus, power consumed by abacklight can be reduced, so that the power consumption of thesemiconductor device can be reduced.

In the semiconductor device, a channel formation region of thetransistor can have an oxide semiconductor layer. A transistor includingan oxide semiconductor has an electrical characteristic of much loweroff-state current than a transistor including silicon or the like.Therefore, when a transistor including an oxide semiconductor is used asthe switching transistor in the pixel, an image signal written to thedisplay element can be held for a long period without a change in thecircuit structure or the like of the pixel. Accordingly, in the casewhere still images or the like is displayed, write frequency can belowered. Thus, power consumption can be reduced.

In one embodiment of the present invention, signals are transmitted andreceived wirelessly (without contact). Therefore, a technique by which atransistor including an oxide semiconductor is used as a switchingtransistor in a pixel and write frequency and transmitting and receivingspeed of signals can be lowered as described above is very useful in oneembodiment of the present invention. With the transistor, an imagesignal written to a display element can be held for a long period.Accordingly, even in the case where write frequency is low, degradation(change) in display of the pixel can be suppressed.

In one embodiment of the present invention, the problem of contactfailure generated in an FPC terminal portion can be solved when signalsand power are supplied wirelessly without the use of an FPC. Inaddition, even in the case where signals and power are suppliedwirelessly, the signals and power can be received highly efficiently.Further, even in the case where vibration is caused or the temperatureis changed, the distance between antennas can be kept constant, so thatsignals and power can be received highly efficiently.

Furthermore, even in the case where a flexible substrate is used as asubstrate, the distance between antennas can be kept constant, so thatsignals and power can be received highly efficiently. Therefore, asemiconductor device can have various structures when the substrate isbent.

Moreover, when a transistor including an oxide semiconductor is used asa switching transistor included in a pixel portion, an image signalwritten to a display element can be held for a long period. Therefore,even in the case where signals and power are supplied wirelessly,high-quality images can be displayed.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is an example of a top view of a semiconductor device;

FIGS. 2A and 2B are examples of top views of a semiconductor device, andFIGS. 2C to 2E are examples of cross-sectional views of thesemiconductor device;

FIG. 3A is an example of a perspective view of the semiconductor device,and FIG. 3B is an example of a cross-sectional view of the semiconductordevice;

FIG. 4 is an example of a block diagram of the semiconductor device;

FIG. 5 is an example of a waveform of a signal input to thesemiconductor device;

FIG. 6 is an example of a block diagram of the semiconductor device;

FIGS. 7A and 7B are examples of a structure of a pixel portion includedin the semiconductor device;

FIGS. 8A to 8C are schematic views illustrating leak paths of imagesignals in the pixel portion included in the semiconductor device;

FIGS. 9A to 9D are examples of a structure of a transistor included inthe semiconductor device and a manufacturing method thereof;

FIG. 10 is an example of a top view of a semiconductor device;

FIG. 11 illustrates examples of V_(g)−I_(d) characteristics of a testelement group of a transistor included in a semiconductor device;

FIGS. 12A and 12B are examples of top views of the test element group ofthe transistor included in the semiconductor device;

FIGS. 13A and 13B are examples of V_(g)−I_(d) characteristics of thetest element group of the transistor included in the semiconductordevice;

FIG. 14 is an example of a diagram illustrating a circuit for evaluatingcharacteristics of the transistor included in the semiconductor device;

FIG. 15 illustrates examples of the characteristics of the transistorincluded in the semiconductor device; and

FIG. 16 illustrates examples of the characteristics of the transistorincluded in the semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments and examples of the present invention will be describedbelow with reference to the drawings. Note that the present invention isnot limited to the following description. It will be readily appreciatedby those skilled in the art that modes and details of the presentinvention can be changed in various ways without departing from thespirit and scope of the present invention. Therefore, the presentinvention should not be construed as being limited to the followingdescription of the embodiments and examples. Note that in thedescription of the structure of the present invention with reference tothe drawings, reference numerals denoting the same components are usedin common in different drawings.

Embodiment 1

In this embodiment, an example of a semiconductor device which is oneembodiment of the disclosed invention is described with reference toFIG. 1 and FIGS. 2A to 2E. In this embodiment, an example in which asemiconductor device is a display is described.

FIG. 1 is an example of the top view of a substrate 301 included in thesemiconductor device described in this embodiment. The substrate 301 isprovided with a signal antenna 305 and a power antenna 306. The signalantenna 305 and the power antenna 306 are used as receiver antennas. Inaddition, a signal processing portion 307 and a power source portion 308are provided so as to be electrically connected to the antennas.

The semiconductor device described in this embodiment is a display,which includes a pixel portion 302 on the substrate 301. The pixelportion 302 includes a plurality of pixels. Further, in order to drivethe plurality of pixels included in the pixel portion 302, a scan linedriver circuit 303 and a signal line driver circuit 304 are provided.Note that a side on which the antennas and the like are provided isreferred to as a surface side of the substrate.

FIGS. 2A to 2E are examples of top views and cross-sectional views ofthe substrate 301 and a substrate 601 included in the semiconductordevice described in this embodiment.

FIG. 2A is an example of the top view of the substrate 301. Thestructure in FIG. 2A is substantially the same as the structure inFIG. 1. That is, the substrate 301 is provided with the signal antenna305 and the power antenna 306. The signal antenna 305 and the powerantenna 306 are used as receiver antennas. In addition, the signalprocessing portion 307 and the power source portion 308 are provided soas to be electrically connected to the antennas. Further, the substrate301 is provided with the pixel portion 302. Although FIG. 2A does notillustrate a scan line driver circuit and a signal line driver circuit,the semiconductor device illustrated in FIG. 2A can include a scan linedriver circuit and a signal line driver circuit as in FIG. 1.

FIG. 2B is an example of the top view of the substrate 601. Thesubstrate 601 is provided with a signal antenna 605 and a power antenna606. The signal antenna 605 and the power antenna 606 are used astransmitter antennas. In addition, an integrated circuit 602 is providedso as to be electrically connected to the antennas. Further, signals andpower needed in the integrated circuit 602 are input from the outside.Signals and power needed in the integrated circuit 602 can be suppliedwirelessly. In that case, in addition to the signal antenna 605 and thepower antenna 606, another antenna may be provided. Alternatively,signals and power needed in the integrated circuit 602 can be input fromthe outside through an FPC or the like.

FIG. 2C illustrates an A-A′ cross section of the substrate 301 in FIG.2A. FIG. 152D illustrates a B-B′ cross section of the substrate 601 inFIG. 2B. FIG. 2C is an example of a cross-sectional view of thesubstrate 301 and the substrate 601 before being attached to each other.FIGS. 2D and 2E are examples of a cross-sectional view of the substrate301 and the substrate 601 after being attached to each other.

As illustrated in FIG. 2C, the signal antenna 305 is provided in theA-A′ cross section of the substrate 301. In addition, a substrate 331 isprovided over the pixel portion 302 of the substrate 301. The substrate331 is used as a substrate provided with a counter electrode facing apixel electrode, a substrate which protects the pixel portion 302, or asubstrate which seals the pixel portion 302. The signal antenna 605 isprovided in the B-B′ cross section of the substrate 601.

FIG. 2D is the example of the cross-sectional view of the substrate 301and the substrate 601 after being attached to each other. As illustratedin FIG. 2D, the substrate 601 is attached on a back side of thesubstrate 301. The substrate 601 is attached on the back side of thesubstrate 301 so that a side on which the signal antenna 605 and thelike are provided corresponds to the substrate 301 side. At the time ofthe attachment, the substrate 301 and the substrate 601 are provided sothat the signal antenna 305 and the signal antenna 605 overlap with eachother when viewed from above. In addition, the substrate 301 and thesubstrate 601 are provided so that the power antenna 306 and the powerantenna 606 overlap with each other when viewed from above. In thismanner, the signal antenna 305 and the signal antenna 605 overlap witheach other with the substrate 301 provided therebetween to be fixed toeach other. Further, the power antenna 306 and the power antenna 606overlap with each other with the substrate 301 provided therebetween tobe fixed to each other. Further, as illustrated in FIG. 2E, thesubstrate 301 may be attached on a back side of the substrate 601.

When the receiver antennas (the signal antenna 305 and the power antenna306) and the transmitter antennas (the signal antenna 605 and the powerantenna 606) overlap with each other with the substrate 301 or thesubstrate 601 provided therebetween to be fixed to each other in thismanlier, signals and power can be received highly efficiently. Whenreception efficiency is made high, a phenomenon that communicationfailure occurs only in the case of certain field intensity, i.e., adropout phenomenon can be prevented.

Although not illustrated in FIGS. 2C to 2E, over the substrate 601, aninsulating film can be provided over the signal antenna 605 and thepower antenna 606. Similarly, over the substrate 301, an insulating filmcan be provided over the signal antenna 305 and the power antenna 306.These insulating films can function as protective films. Further, theseinsulating films can have a function of flattening surfaces of thesubstrates. In the case where the insulating film is provided over thepower antenna 606, the insulating film can be used as a bond surface.

The substrate 301 and the substrate 601 can be attached to each otherwith an adhesive or the like. A material which firmly attaches a bondsurface of the substrate 301 and a bond surface of the substrate 601 toeach other can be used as the material of the adhesive. In addition, amaterial which can make the thickness of a layer of the adhesive(referred to as an adhesion layer) small can be used. When the adhesionlayer is thin, the thickness of the adhesion layer can be made uniformin a plane.

In addition, a material into which an insulating filler is mixed may beused as the adhesive. When a material into which an insulating filler ismixed is used as the adhesive, the thickness of the adhesion layer canbe further made uniform.

A region where the substrate 301 and the substrate 601 are attached toeach other can be a region including a region where the antennas areprovided over the substrate 301 and the substrate 601. For example, inthe case of the substrate 301, a diagonally shaded region 341 shown inFIG. 2A can be used as an attachment region. In the case of thesubstrate 601, a diagonally shaded region 641 shown in FIG. 2B can beused as an attachment region. In this manner, the area of the regionwhere the substrate 301 and the substrate 601 are attached to each othercan be the same as or larger than the area of the antennas. The antennashave a certain size. Thus, the attachment portion has a certain size.Accordingly, the bond strength of the attachment portion can be madehigh.

The thickness of the substrate 301 can be in the range of from 0.1 to3.0 mm. Thus, signals and power can be received highly efficiently.Further, a phenomenon that communication failure occurs only when thetransmitter antenna and the receiver antenna are placed in a certaindistance, i.e., a dropout phenomenon can be prevented. This is becausethe distance between the signal antenna 305 and the signal antenna 605provided on the surface side and back side of the substrate 301 and thedistance between the power antenna 306 and the power antenna 606provided on the surface side and back side of the substrate 301 aredetermined mainly by the thickness of the substrate 301. In the casewhere the thickness of the substrate 301 exceeds the above range, thedropout phenomenon might occur. However, the thickness of the substrate301 is not necessarily limited to the above range. The substrate 301 canhave thickness which is beyond the above range as long as signals andpower can be transmitted and received and the dropout phenomenon doesnot occur.

Note that although the signal antenna 305 and the signal processingportion 307 are separately illustrated in FIG. 1 and FIG. 2A, the signalantenna 305 may be included in the signal processing portion 307. Inaddition, although the power antenna 306 and the power source portion308 are separately illustrated in FIG. 1 and FIG. 2A, the power antenna306 may be included in the power source portion 308. Further, the shapeof the antenna is not limited to a spiral shape. A rod shape, a loopshape, or the like can be used.

In this embodiment, a set of the signal antenna 305 (the receiverantenna) and the signal antenna 605 (the transmitter antenna) isprovided; however, this embodiment is not limited to this. Plural setsof the signal antenna 305 and the signal antenna 605 can be provided. Inthat case, the plurality of signal antennas 305 (receiver antennas) canbe provided in empty spaces of the substrate 301. In addition, some orall of the plurality of signal antennas 605 (transmitter antennas) canbe provided over the substrate 601 which is provided with the powerantenna 606. In the case where some of the plurality of signal antennas605 (transmitter antennas) are provided over the substrate 601 which isprovided with the power antenna 606, the other signal antennas 605 (thetransmitter antennas) can be provided over a different substrate.

When plural sets of signal antennas 305 (receiver antennas) and signalantennas 605 (transmitter antennas) are provided in this manner, thetransmitting and receiving speed of signals can be improved. Accordingto this embodiment, the problem of bad connection in a portion connectedto the outside is less likely to be caused as compared to the case wherean FPC or the like is used. Accordingly, a structure where the number ofsignal input/output portions (i.e., the signal antenna 305 (the receiverantenna) and the signal antenna 605 (the transmitter antenna)) isincreased can be easily employed.

In this embodiment, the problem of contact failure generated in an FPCterminal portion can be solved when signals and power are suppliedwirelessly without the use of an FPC. In addition, even in the casewhere signals and power are supplied wirelessly, the signals and powercan be received highly efficiently. Further, even in the case wherevibration is caused or the temperature is changed, the distance betweenthe antennas can be kept constant, so that signals and power can bereceived highly efficiently.

The semiconductor device described in this embodiment can be used as adisplay in a variety of devices such as a liquid crystal television, apersonal computer, a cellular phone, an e-book reader, a digital camera,a personal digital assistant, and portable audio equipment.

The semiconductor device described in this embodiment is resistant tovibration or the change in temperature; thus, the semiconductor devicecan be used as a versatile display. For example, the semiconductordevice can be used as a display provided in a vehicle such as a railroadtrain, an electric train, a car, a ship, or an airplane. In addition,the semiconductor device can be used as a display provided on a wall ora column of a construction such as a station or a building. Further, thesemiconductor device can be used as a display provided in a portabledevice such as a cellular phone, an e-book reader, or a personal digitalassistant. Furthermore, the semiconductor device can be used as a devicehaving a waterproof function.

Moreover, the semiconductor device described in this embodiment can beused not only as a display but also an electronic component or anelectronic device.

Further, the number of power lines is small. For example, the number ofpower lines per substrate can be two. Since the number of power lines issmall in this manner, in the power source portion, a wiring providedover the substrate 301 can be directly connected to a wiring providedover the substrate 601. In that case, the power lines can be connectedto an external terminal with the use of an FPC or the like. Accordingly,in the semiconductor device, it is possible not to provide the powerantenna 306 and the power antenna 606. Also in that case, the number ofsignal lines is large; thus, the problem of contact failure generated inthe FPC terminal portion can be solved when signals are suppliedwirelessly. In addition, even in the case where signals are suppliedwirelessly, the signals can be received highly efficiently. Further,even in the case where vibration is caused or the temperature ischanged, the distance between the antennas can be kept constant, so thatsignals can be received highly efficiently.

This embodiment can be combined with any of the other embodiments andexamples as appropriate.

Embodiment 2

An example of a semiconductor device which is one embodiment of thedisclosed invention is described with reference to FIGS. 3A and 3B. Inthis embodiment, an example in which a substrate is a flexible substrateis described. Further, in this embodiment, an example in which asemiconductor device is a display is described.

FIGS. 3A and 3B are examples of the perspective view and thecross-sectional view of the substrate 301 and the substrate 601 whichare included in the semiconductor device described in this embodiment.

FIG. 3A is an example of the perspective view of the substrate 301 andthe substrate 601. The structure in FIG. 3A is substantially the same asthe structure in FIG. 1, the structure in FIG. 2A, and the structure inFIG. 2B. That is, the substrate 301 illustrated in FIG. 3A is providedwith the signal antenna 305 and the power antenna 306. The signalantenna 305 and the power antenna 306 are used as receiver antennas. Inaddition, the signal processing portion 307 and the power source portion308 are provided so as to be electrically connected to the antennas.Further, the substrate 301 is provided with the pixel portion 302.Although FIG. 3A does not illustrate a scan line driver circuit and asignal line driver circuit, the semiconductor device illustrated in FIG.3A can include a scan line driver circuit and a signal line drivercircuit as in FIG. 1.

The substrate 601 illustrated in FIG. 3A is provided with the signalantenna 605 and the power antenna 606. The signal antenna 605 and thepower antenna 606 are used as transmitter antennas. In addition, theintegrated circuit 602 is provided so as to be electrically connected tothe antennas.

FIG. 3B is an example of the cross-sectional view of the substrate 301and the substrate 601 after being attached to each other. As illustratedin FIG. 3B, the substrate 601 is attached on a back side of thesubstrate 301. The substrate 601 is attached on the back side of thesubstrate 301 so that a side on which the signal antenna 605 and thelike are provided corresponds to the substrate 301 side. At the time ofthe attachment, the substrate 301 and the substrate 601 are provided sothat the signal antenna 305 and the signal antenna 605 overlap with eachother when viewed from above. In addition, the substrate 301 and thesubstrate 601 are provided so that the power antenna 306 and the powerantenna 606 overlap with each other when viewed from above. In thismanner, the receiver antennas (the signal antenna 305 and the powerantenna 306) and the transmitter antennas (the signal antenna 605 andthe power antenna 606) overlap with each other with the substrate 301provided therebetween to be fixed to each other.

In this embodiment, flexible substrates are used as the substrate 301and the substrate 601. A flexible substrate is a substrate that can bebent (is flexible), for example, a plastic substrate includingpolycarbonate, polyarylate, or polyether sulfone, or the like.Alternatively, a film (a film including polypropylene, polyester, vinyl,polyvinyl fluoride, vinyl chloride, or the like), an inorganic vapordeposition film, or the like can be used.

The flexible substrate is bent in some cases. Thus, for example, in thecase where the receiver antenna is provided over the flexible substrateand the transmitter antenna is provided over a different substrate, thedistance between the receiver antenna and the transmitter antenna cannotbe kept constant in some cases. However, in this embodiment, thereceiver antennas (the signal antenna 305 and the power antenna 306) andthe transmitter antennas (the signal antenna 605 and the power antenna606) are fixed to the substrate 301. Thus, even in the case where thesubstrate 301 is bent as illustrated in FIG. 3B, the distance betweenthe receiver antenna and the transmitter antenna can be kept constant.Accordingly, even in the case where the substrate 301 is bent, signalsand power can be received highly efficiently.

In this manner, in the semiconductor device of this embodiment, thesubstrate 301 can be bent. The semiconductor device can have variousstructures when the substrate 301 is bent. Any substrate can be used asthe substrate 301 as long as it has a certain thickness and includes amaterial which transmits an electromagnetic wave with frequency used fortransmission and reception of signals and power. An insulating materialcan be used as a material which transmits an electromagnetic wave withfrequency used for transmission and reception of signals and power. Inaddition, a flexible substrate can be used as the substrate 601 attachedto the substrate 301. When a flexible substrate is used as the substrate601, in the case where the substrate 301 is bent, the substrate 601 isbent similarly. Therefore, even in the case where the substrate is bent,the distance between the antennas can be kept constant.

In this embodiment, the problem of contact failure generated in an FPCterminal portion can be solved when signals and power are suppliedwirelessly without the use of an FPC. In addition, even in the casewhere signals and power are supplied wirelessly, the signals and powercan be received highly efficiently. Further, even in the case wherevibration is caused or the temperature is changed, the distance betweenthe antennas can be kept constant, so that signals and power can bereceived highly efficiently.

Furthermore, even in the case where a flexible substrate is used as thesubstrate, the distance between the antennas can be kept constant, sothat signals and power can be received highly efficiently. Therefore,the semiconductor device can have various structures when the substrateis bent.

This embodiment can be combined with any of the other embodiments andthe examples as appropriate.

Embodiment 3

In this embodiment, examples of the structure and operation of asemiconductor device which is one embodiment of the disclosed inventionare described with reference to FIG. 4 and FIG. 5. In this embodiment,an example in which a semiconductor device is a display is described.

FIG. 4 is an example of the block diagram of the semiconductor devicedescribed in this embodiment. Here, the substrate 301 included in thesemiconductor device is described.

As illustrated in FIG. 4, the substrate 301 includes the signalprocessing portion 307 having the signal antenna 305, the power sourceportion 308 having the power antenna 306, the pixel portion 302, and thescan line driver circuit 303 and the signal line driver circuit 304 fordriving the pixel portion 302.

The signal processing portion 307 includes the signal antenna 305, ademodulation circuit 311, a clock generator 312, a signal processingcircuit 313, a memory circuit 314, a memory circuit 315, a displaycontroller 316, and the like. The power source portion 308 includes thepower antenna 306, a rectifier circuit 321, a battery (or a capacitor)322, a DC-DC converter 323, and the like. The signal antenna 305 and thepower antenna 306 are used as receiver antennas.

FIG. 5 shows the waveform of a signal input to the signal antenna 305included in the semiconductor device described in this embodiment. Thesignal is modulated and includes a modulated wave 702 and anon-modulated wave 701. The reliability of the modulated wave can beimproved when the modulated wave is encoded to be transmitted.Manchester encoding, deformable mirrors, NRZ, or the like can be used asthe encoding method; however, this embodiment is not limited to this.

In addition, 13.56 MHz can be used as the frequency of the non-modulatedwave 701; however, the frequency of the non-modulated wave 701 is notlimited to this frequency. The amount of data can be increased when thefrequency is made high.

Next, the operation of the semiconductor device in this embodiment isdescribed. A signal input to the signal antenna 305 is input to thedemodulation circuit 311 and the clock generator 312. A modulated wave(the modulated wave 702 shown in FIG. 5) is demodulated in thedemodulation circuit 311. The demodulation circuit 311 includes arectifier circuit having a diode, for example; however, this embodimentis not limited to this. Further, the clock generator 312 generates aclock signal with the use of a non-modulated wave (the non-modulatedwave 701 shown in FIG. 5). The clock signal may have the frequency ofthe non-modulated wave (the non-modulated wave 701 shown in FIG. 5) orfrequency which is lowered using a frequency divider.

The demodulated signal and the clock signal are input to the signalprocessing circuit 313 and decoded to be original image signals. Theimage signals are input to the memory circuit 314, the memory circuit315, and the display controller 316. From the image signals, the displaycontroller 316 outputs clock signals, start pulses, latch pulses, andthe like for the scan line driver circuit 303 and the signal line drivercircuit 304 which drive the pixel portion 302. Further, the signalprocessing circuit 313 extracts data to be input to the pixel portion302 from the image signals and inputs the data to the memory circuit 314and the memory circuit 315. Two memory circuits are provided in orderthat while the transmitted data is stored in one memory, data be readfrom the other memory to be displayed. When subsequent data is stored,the memory for storing the data and the memory for reading the data maybe interchanged with each other.

Then, the power source portion 308 is described. The power sourceportion 308 includes the power antenna 306, the rectifier circuit 321,the battery (or the capacitor) 322, the DC-DC converter 323, and thelike. A demodulation circuit having a diode is generally used for therectifier circuit 321; however, this embodiment is not limited to this.Rectified voltage is stored in the battery (or the capacitor) 322. Then,power (also referred to as power supply voltage) is supplied to thesignal processing portion 307, the scan line driver circuit 303, and thesignal line driver circuit 304 through the DC-DC converter 323.Frequency for power supply does not necessarily correspond to frequencyfor signal supply. The frequencies may be different from each other.

Circuits such as the demodulation circuit 311, the clock generator 312,the signal processing circuit 313, the memory circuit 314, the memorycircuit 315, the display controller 316, the rectifier circuit 321, andthe DC-DC converter 323 may each include a transistor with the samestructure as the transistor included in each of the plurality of pixelsin the pixel portion 302, may include a transistor with a structurewhich is different from the structure of the transistor included in thepixel, or may be provided with an IC chip.

The substrate 601 illustrated in FIGS. 2A to 2E and FIGS. 3A and 3B canbe attached to the substrate 301 included in the semiconductor devicedescribed in this embodiment.

In this embodiment, the problem of contact failure generated in an FPCterminal portion can be solved when signals and power are suppliedwirelessly without the use of an FPC. In addition, even in the casewhere signals and power are supplied wirelessly, the signals and powercan be received highly efficiently. Further, even in the case wherevibration is caused or the temperature is changed, the distance betweenthe antennas can be kept constant, so that signals and power can bereceived highly efficiently.

This embodiment can be combined with any of the other embodiments andthe examples as appropriate.

Embodiment 4

In this embodiment, an example of a semiconductor device which is oneembodiment of the disclosed invention is described with reference toFIG. 6. In this embodiment, an example in which one antenna is used as asignal antenna and a power antenna is described. Further, in thisembodiment, an example in which a semiconductor device is a display isdescribed.

FIG. 6 is an example of the block diagram of the semiconductor devicedescribed in this embodiment. Here, the substrate 301 included in thesemiconductor device is described.

As illustrated in FIG. 6, the substrate 301 includes an antenna 335, thesignal processing portion 307, the power source portion 308, the pixelportion 302, and the scan line driver circuit 303 and the signal linedriver circuit 304 for driving the pixel portion 302.

The antenna 335 serves as both a signal antenna and a power antenna. Theantenna 335 is used as a receiver antenna. When one antenna is used as asignal antenna and a power antenna in this manlier, a space in which anantenna is provided can be saved. In that case, frequency for powersupply and frequency for signal supply are the same.

The structures and operations (except the structure and operation of theantenna 335) are similar to those in FIG. 4.

The substrate 601 illustrated in FIGS. 2A to 2E and FIGS. 3A and 3B canbe attached to the substrate 301 included in the semiconductor devicedescribed in this embodiment.

In this embodiment, the problem of contact failure generated in an FPCterminal portion can be solved when signals and power are suppliedwirelessly without the use of an FPC. In addition, even in the casewhere signals and power are supplied wirelessly, the signals and powercan be received highly efficiently. Further, even in the case wherevibration is caused or the temperature is changed, the distance betweenthe antennas can be kept constant, so that signals and power can bereceived highly efficiently. Furthermore, a space in which an antenna isprovided can be saved.

This embodiment can be combined with any of the other embodiments andthe examples as appropriate.

Embodiment 5

In this embodiment, examples of a semiconductor device which is oneembodiment of the disclosed invention are described with reference toFIGS. 7A and 7B and FIGS. 8A to 8C. In this embodiment, examples of apixel portion included in the semiconductor device and a driver circuitwhich drives the pixel portion when the semiconductor device is adisplay are described. Specifically, in this embodiment, examples of thepixel portion and the driver circuit which drives the pixel portion whenthe semiconductor device is an active-matrix liquid crystal displaywhere transistors are arranged in matrix in the pixel portion aredescribed with reference to FIGS. 7A and 7B and FIGS. 8A to 8C. In thisembodiment, a liquid crystal element is used as a display element.

FIG. 7A illustrates a structure example of the liquid crystal display.As illustrated in FIG. 7A, the liquid crystal display includes the scanline driver circuit 303, the signal line driver circuit 304, and thepixel portion 302. The pixel portion 302 includes a plurality of pixels14 arranged in matrix. FIG. 7B illustrates a structure example of thepixel. The pixel 14 illustrated in FIG. 7B includes a transistor 15, aliquid crystal element 16, and a capacitor 17. A gate terminal of thetransistor 15 is electrically connected to the scan line driver circuit303. A first terminal of the transistor 15 is electrically connected tothe signal line driver circuit 304. One terminal of the liquid crystalelement 16 is electrically connected to a second terminal of thetransistor 15. The other terminal of the liquid crystal element 16 iselectrically connected to a wiring for supplying a common potential(V_(com)). One terminal of the capacitor 17 is electrically connected tothe second terminal of the transistor 15 and the one terminal of theliquid crystal element 16. The other terminal of the capacitor 17 iselectrically connected to a wiring for supplying the common potential(V_(com)). For the scan line driver circuit 303 and the signal linedriver circuit 304, transistors provided over the substrate included inthe semiconductor device that are similar to the transistors 15 in thepixel portion may be used, or an IC chip may be attached onto thesubstrate included in the semiconductor device by chip on glass (COG).

In the liquid crystal display of this embodiment, on/off of thetransistor 15 is controlled by the scan line driver circuit 303, and animage signal is input to the liquid crystal element 16 from the signalline driver circuit 304 through the transistor 15. Note that the liquidcrystal element 16 includes a liquid crystal layer held between the oneterminal and the other terminal. Voltage which corresponds to adifference between the potential of the image signal and the commonpotential (V_(com)) is applied to the liquid crystal layer and is usedfor control of the alignment of the liquid crystal layer. In the liquidcrystal display of this embodiment, display of the pixel 14 iscontrolled utilizing the alignment. Note that the capacitor 17 isprovided in order to hold voltage applied to the liquid crystal element16.

Further, in the liquid crystal display described in this embodiment,when the operation of the scan line driver circuit 303 and the signalline driver circuit 304 is controlled by the display controller 316,input of an image signal to the pixel portion 302 can be selected.

<Transistor>

The transistor 15 is a transistor whose channel formation regionincludes an oxide semiconductor layer. The oxide semiconductor layer isan oxide semiconductor layer which is high-purity and is made to beelectrically i-type (intrinsic) or substantially i-type (intrinsic) bydrastic removal of an impurity that causes variation in electricalcharacteristics of the transistor, such as hydrogen, moisture, ahydroxyl group, or hydride and by supply of oxygen which is a maincomponent of the oxide semiconductor that is simultaneously reduced in astep of removing the impurity. Note that the oxide semiconductorincluded in the oxide semiconductor layer has a band gap of 3.0 eV ormore.

Further, the number of carriers in the high-purity oxide semiconductoris significantly small (close to zero), and the carrier density of theoxide semiconductor is significantly low (e.g., lower than 1×10¹²/cm³,preferably lower than 1×10¹¹/cm³). Thus, the off-state current of thetransistor is significantly low. Therefore, in the transistor, off-statecurrent per micrometer of the channel width (W) at room temperature canbe 1 aA/μm (1×10⁻¹⁸ A/μm) or less, or less than 100 zA/μm (1×10⁻¹⁹A/μm). Note that in general, in the case of a transistor includingamorphous silicon, off-state current at room temperature is 1×10⁻¹³ A/μmor more. Further, hot carrier degradation does not occur in thetransistor. Accordingly, the electrical characteristics of thetransistor are not adversely affected by hot carrier degradation.

Thus, an image signal can be held in each of the pixels 14 for a longerperiod. That is, an interval between rewrites of image signals whenstill images are displayed can be extended. For example, an intervalbetween writes of image signals can be 10 seconds or longer, preferably30 seconds or longer, more preferably 1 minute or longer and shorterthan 10 minutes. When the interval between writes of image signals isextended, power consumption can be reduced by the extended interval.

Note that the resistance to flow of the off-state current of atransistor can be referred to as off-state resistivity. The off-stateresistivity is resistivity of a channel formation region when thetransistor is off, and the off-state resistivity can be calculated fromthe off-state current.

Specifically, if the amount of off-state current and the level of drainvoltage are known, resistance when the transistor is off (off resistanceR) can be calculated using Ohm's law. In addition, if a cross section Aof the channel formation region and the length L of the channelformation region (the length corresponds to a distance between a sourceelectrode and a drain electrode) are known, off-state resistivity ρ canbe calculated from the formula ρ=RAIL (R is off resistance).

Here, the cross section A can be calculated from the formula A=dW (d isthe thickness of the channel formation region and W is the channelwidth). In addition, the length L of the channel formation region ischannel length L. In this manner, the off-state resistivity can becalculated from the off-state current.

The off-state resistivity of the transistor including an oxidesemiconductor layer in this embodiment is preferably 1×10¹¹ Ω·cm (100GΩ·cm) or more, more preferably 1×10¹² Ω·cm (1 TΩ·cm) or more.

By drastically removing hydrogen contained in an oxide semiconductorlayer as described above, in a transistor which includes a high-purityoxide semiconductor layer in a channel formation region, the amount ofoff-state current can be significantly reduced. In other words, incircuit design, the oxide semiconductor layer can be regarded as aninsulator when the transistor is off (non-conducting). In contrast, thecurrent supply capability of a transistor which includes an oxidesemiconductor in a channel formation region is expected to be higherthan that of a transistor including amorphous silicon when thetransistor is on (conducting).

A transistor including low-temperature polysilicon is designed on theassumption that off-state current at room temperature is about 10000times that of a transistor including an oxide semiconductor. Therefore,in the case where the transistor including an oxide semiconductor iscompared with the transistor including low-temperature polysilicon, thevoltage hold time of the transistor including an oxide semiconductor canbe extended about 10000 times when storage capacitances are equal orsubstantially equal to each other (about 0.1 pF). Accordingly, stillimages can be displayed even by less frequent writing of image signals.

When the image signal hold time of the pixels 14 is extended asdescribed above, the frequency of supply of image signals to the pixelscan be reduced. In one embodiment of the present invention, signals aretransmitted and received wirelessly (without contact). Therefore, atechnique by which a transistor including an oxide semiconductor is usedas a transistor in a pixel and write frequency and transmitting andreceiving speed of signals can be lowered as described above is veryuseful in one embodiment of the present invention. Display degradation(change) in the pixel can be suppressed when the transistor is used as atransistor for controlling input of an image signal to the pixel.

Further, when the transistor is used as a switch for controlling inputof an image signal to a pixel, the size of a capacitor provided in thepixel can be made small. Thus, the aperture ratio of the pixel can beimproved and an image signal can be input to the pixel at high speed,for example.

Note that in this specification, a semiconductor with a carrierconcentration lower than 1×10¹¹/cm³ is called an intrinsic (i-type)semiconductor, and a semiconductor with a carrier concentration higherthan or equal to 1×10¹¹/cm³ and lower than 1×10¹²/cm³ is called asubstantially intrinsic (substantially i-type) semiconductor.

<Liquid Crystal Element and Capacitor>

In the case where the transistor is used as the transistor 15 forcontrolling input of an image signal, it is preferable that a substancewith high specific resistivity be used as the liquid crystal material ofthe liquid crystal element 16. Here, the reason for using a substancewith high specific resistivity is described with reference to FIGS. 8Ato 8C. Note that FIG. 8B is a schematic view for illustrating the leakpath of an image signal in a pixel which includes a transistor includingamorphous silicon and the leak path of an image signal in a pixel whichincludes the transistor including an oxide semiconductor.

As illustrated in FIG. 7B, the pixel includes the transistor 15, theliquid crystal element 16, and the capacitor 17. The circuit illustratedin FIG. 7B is equivalent to a circuit illustrated in FIG. 8A when thetransistor 15 is off. That is, the circuit illustrated in FIG. 7B isequivalent to a circuit in which the transistor 15 is assumed to be aresistor (R_(Tr-Off)), and the liquid crystal element 16 is assumed toinclude a resistor (R_(LC)) and a capacitor (C_(LC)). When an imagesignal is input to the pixel, the image signal is stored in thecapacitor 17 (C_(S)) and the capacitor of the liquid crystal element 16(C_(LC)) (see FIG. 7B and FIG. 8A). Then, when the transistor 15 isturned off, the image signal leaks through the transistor 15 and theliquid crystal element 16, as illustrated in FIGS. 8B and 8C. Note thatFIG. 8B is a schematic view illustrating leak of an image signal whenthe transistor is a transistor 25 including amorphous silicon, and FIG.8C is a schematic view illustrating leak of an image signal when thetransistor is the transistor 15 including an oxide semiconductor. Theoff-state resistance of the transistor 25 including amorphous silicon islower than the resistance of the liquid crystal element. Therefore, theimage signal leaks mainly through the transistor 25 including amorphoussilicon, as illustrated in FIG. 8B (i.e., the image signal leaks mainlythrough a path A and a path B in FIG. 8B). In contrast, the off-stateresistance of the transistor 15 including a high-purity oxidesemiconductor is higher than the resistance of the liquid crystalelement. Therefore, the image signal leaks mainly through the liquidcrystal element, as illustrated in FIG. 8C (i.e., the image signal leaksmainly through a path C and a path D in FIG. 8C).

In other words, although characteristics of a transistor provided ineach pixel of a liquid crystal display have been conventionally arate-controlling point in image signal holding characteristics in eachpixel, when the transistor 15 including a high-purity oxidesemiconductor is used as a transistor provided in each pixel, arate-controlling point therein is shifted to the resistance of a liquidcrystal element. Therefore, it is preferable that a substance with highspecific resistivity be used as the liquid crystal material of theliquid crystal element 16.

Specifically, in a liquid crystal display device whose pixel is providedwith the transistor 15 including a high-purity oxide semiconductor, thespecific resistivity of a liquid crystal material is preferably 1×10¹²Ω·cm (1 TΩ·cm) or higher, more preferably higher than 1×10¹² Ω·cm (10TΩ·cm), still preferably higher than 1×10¹⁴ Ω·cm (100 TΩ·cm). Thespecific resistance in this specification is measured at 20° C.

In the still image hold period, the other terminal of the liquid crystalelement 16 can be made to be in a floating state without being suppliedwith the common potential (V_(com)). Specifically, a switch may beprovided between the terminal and a power source for supplying thecommon potential (V_(com)). The switch may be turned on in a writingperiod so that the common potential (V_(com)) may be supplied from thepower source. Then, the switch may be turned off in the remaining holdperiod and the terminal may be made to be in a floating state. It ispreferable that the transistor including a high-purity oxidesemiconductor be used for the switch. When the other terminal of theliquid crystal element 16 is made to be in a floating state, displaydegradation (change) in the pixel 14 due to an irregular pulse or thelike can be suppressed. The reason is described as follows. When thepotential of the first terminal of the transistor 15 which is offfluctuates by an irregular pulse, the potential of the one terminal ofthe liquid crystal element 16 also fluctuates by capacitive coupling. Atthis time, if the common potential (V_(com)) is supplied to the otherterminal of the liquid crystal element 16, the fluctuation in potentialis directly linked to the change in voltage applied to the liquidcrystal element 16. When the other terminal of the liquid crystalelement 16 is in a floating state, the potential of the other terminalfluctuates by capacitive coupling. Accordingly, even when the potentialof the first terminal of the transistor 15 fluctuates by an irregularpulse, the change in voltage applied to the liquid crystal element 16can be reduced. Therefore, display degradation (change) in the pixel 14can be suppressed.

The capacitance of the capacitor 17 (C_(S)) is set in consideration ofthe off-state current of a transistor in each pixel, or the like. Notethat a variety of numeric values in the above description are estimates.

A reflective liquid crystal element is preferably used as a liquidcrystal element used in this embodiment. A reflective liquid crystalelement can be obtained by formation of a reflective electrode as apixel electrode. Thus, power consumed by a backlight can be reduced, sothat the power consumption of the semiconductor device can be reduced.

In addition, although an example in which a liquid crystal element isused as a display element is described in this embodiment, alight-emitting element or an electrophoretic element can be used insteadof the liquid crystal element. When an electrophoretic element is used,power consumption can be reduced. Further, when an electrophoreticelement or a light-emitting element is used, a substrate which can bebent can be employed easily.

The transistor including an oxide semiconductor in this embodiment hasan electrical characteristic of much lower off-state current than atransistor including silicon or the like. Therefore, when the transistorincluding an oxide semiconductor in this embodiment is used as atransistor in a pixel portion, an image signal written to the displayelement can be held for a long period without a change in the circuitstructure or the like of the pixel. Accordingly, in the case where stillimages or the like is displayed, write frequency can be lowered. Thus,power consumption can be reduced.

With the transistor, an image signal written to the display element canbe held for a long period. Accordingly, even in the case where writefrequency is low, degradation (change) in display of the pixel can besuppressed.

A substrate including the pixel portion and the like in this embodimentcan be used as the substrate 301 illustrated in FIG. 1, FIGS. 2A to 2E,FIGS. 3A and 3B, FIG. 4, and FIG. 6. That is, the substrate 601illustrated in FIGS. 2A to 2E and FIGS. 3A and 3B can be attached to thesubstrate including the pixel portion and the like in this embodiment.

This embodiment can be combined with any of the other embodiments andthe examples as appropriate.

Embodiment 6

In this embodiment, an example of the operation of a semiconductordevice which is one embodiment of the disclosed invention is describedwith reference to FIG. 4 and FIG. 5. In this embodiment, therelationship between the frequency of an image signal and imageprocessing is described.

FIG. 5 shows the waveform of a signal input to the signal antenna 305included in the semiconductor device described in this embodiment. Thesignal is modulated and includes the modulated wave 702 and thenon-modulated wave 701.

13.56 MHz can be used as the frequency of the non-modulated wave 701. Inthat case, it is preferable that the modulated wave have a frequencyless than or equal to ⅛ the frequency of the non-modulated wave. Whenthe frequency of the modulated wave is ⅛ the frequency of thenon-modulated wave, the frequency of the modulated wave is 1.695 MHz.When the number of pixels in the pixel portion 302 included in thesemiconductor device is VGA (640×480 dots), images cannot be displayedwithout any operation because the dot clock of VGA is originally 25 MHz.

In addition, when the number of colors in the display is 65500, 16 bitsare needed in each pixel. The frequency is obtained in the case of amonochrome 1 bit. Thus, the frequency is 1/16 in the case of 16 bits, sothat the frequency is 106 kHz. The frequency is 1/236 the dot clock ofVGA (25 MHz).

In general, image signals are written 60 times per second (at 60 fps).However, in the semiconductor device described in this embodiment, writefrequency is decreased to 1/236. Accordingly, an image signal is writtenonce about every four seconds (at about 0.25 fps).

Therefore, in the case where a transistor including amorphous silicon ora transistor including polysilicon is used in the pixel, image signalscannot be held for four seconds. Thus, it is necessary to reduce thenumber of image signals by the decrease in the number of pixels or thedecrease in the number of colors as compared to VGA.

In contrast, in the semiconductor device described in this embodiment,as illustrated in FIGS. 7A and 7B and FIGS. 8A to 8C, a transistorincluding a high-purity oxide semiconductor layer is used as thetransistor included in the pixel. Thus, image signals can be held for along period because the off-state current of the transistor included inthe pixel at room temperature can be 1 aA/μm or lower, or lower than 100zA/μm when the high-purity oxide semiconductor layer is used asdescribed above. For example, in the case where an image with theresolution of VGA or the like is displayed, the image signal can be heldfor 2000 seconds (i.e., 30 minutes or longer) when a transistor whoseoff-state current is 1 aA/μm or lower is used as the transistor in thepixel. Further, an image signal can be held for 20000 seconds (i.e., 330minutes or longer) when a transistor whose off-state current is lowerthan 100 zA/μm is used. Accordingly, in the semiconductor devicedescribed in this embodiment, an image with the resolution of VGA orhigher can be displayed.

According to this embodiment, the problem of contact failure generatedin an FPC terminal portion can be solved when signals and power aresupplied wirelessly without the use of an FPC. In addition, even in thecase where signals and power are supplied wirelessly, the signals andpower can be received highly efficiently. Further, even in the casewhere vibration is caused or the temperature is changed, the distancebetween antennas can be kept constant, so that signals and power can bereceived highly efficiently. Furthermore, an image with the resolutionof VGA or the like can be displayed.

This embodiment can be combined with any of the other embodiments andthe examples as appropriate.

Embodiment 7

In this embodiment, an example of a transistor in a pixel portionincluded in a semiconductor device which is one embodiment of thedisclosed invention is described with reference to FIGS. 9A to 9D.

FIGS. 9A to 9D are cross-sectional views illustrating examples of thestructure of the transistor illustrated in FIGS. 7A and 7B and a methodfor manufacturing the transistor. A transistor 410 illustrated in FIG.9D has a kind of bottom-gate structure called an inverted-staggeredstructure. Further, the transistor 410 has a channel-etched structure.Furthermore, the transistor 410 has a single-gate structure.

However, the structure of the transistor is not limited to the above.The transistor may have a top-gate structure. Further, the transistormay have a channel-stop structure. Furthermore, the transistor may havea multi-gate structure.

Steps of manufacturing the transistor 410 over a substrate 400 aredescribed below with reference to FIGS. 9A to 9D.

First, a gate electrode layer 411 is formed over the substrate 400having an insulating surface (see FIG. 9A).

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least heat resistance high enough towithstand heat treatment to be performed later.

An insulating film serving as a base film may be provided between thesubstrate 400 and the gate electrode layer 411. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 400, and can be formed to have a single-layer structure or alayered structure including one or more films selected from a siliconnitride film, a silicon oxide film, a silicon nitride oxide film, or asilicon oxynitride film. Here, a 100-nm-thick silicon nitride film isformed by plasma-enhanced CVD, and a 150-nm-thick silicon oxynitridefilm (SiON film) is formed over the silicon nitride film byplasma-enhanced CVD.

Note that the base film is preferably formed so as to contain impuritiessuch as hydrogen and water as little as possible.

A conductive layer is formed over the substrate 400 and is selectivelyetched through a first photolithography process, so that the gateelectrode layer 411 can be formed.

The gate electrode layer 411 can be formed to have a single-layerstructure or a layered structure including a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium, or an alloy material which includes any of thesemetal elements as a main component. Here, a 100-nm-thick tungsten filmis formed by sputtering and is etched to be the gate electrode layer411.

Then, a gate insulating layer 402 is formed over the gate electrodelayer 411 (see FIG. 9A).

The gate insulating layer 402 can be formed to have a single-layerstructure or a layered structure including a silicon oxide layer, asilicon nitride layer, a silicon oxynitride layer, a silicon nitrideoxide layer, or an aluminum oxide layer by plasma-enhanced CVD,sputtering, or the like. For example, a silicon oxynitride layer may beformed using silane (SiH₄), oxygen, and nitrogen as a deposition gas byplasma-enhanced CVD. Alternatively, a high-k material such as hafniumoxide (HfO_(x)) or tantalum oxide (TaO_(x)) can be used for the gateinsulating layer. The thickness of the gate insulating layer 402 can be,for example, 10 to 500 nm.

Here, a 30-nm-thick silicon oxynitride film which serves as a gateinsulating layer is formed over the gate electrode layer 411 byhigh-density plasma-enhanced CVD using microwaves (e.g., a frequency of2.45 GHz). The high-density plasma-enhanced CVD using microwaves ispreferable because the dense high-quality insulating layer 402 havinghigh withstand voltage can be formed. When an oxide semiconductor layerand the high-quality gate insulating layer 402 are in close contact witheach other, interface state density can be reduced and interfaceproperties can be favorable.

Note that the gate insulating layer 402 is preferably formed so as tocontain impurities such as hydrogen and water as little as possible.

Then, an oxide semiconductor film 430 is formed over the gate insulatinglayer 402 (see FIG. 9A). The oxide semiconductor film 430 can be formedby sputtering. The thickness of the oxide semiconductor film 430 can be2 to 200 nm.

Note that before the oxide semiconductor film 430 is formed bysputtering, it is preferable to perform reverse sputtering in which anargon gas is introduced and plasma is generated. Powdery substances(also referred to as particles or dust) on a surface of the gateinsulating layer 402 can be removed by the reverse sputtering. Thereverse sputtering is a method in which, without application of voltageto a target side, an RF power source is used for application of voltageto a substrate side and plasma is generated so that a substrate surfaceis modified. Note that nitrogen, helium, oxygen, or the like may be usedinstead of the argon atmosphere.

As the oxide semiconductor film 430, an In—Ga—Zn—O-based material, anIn—Sn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, an In—Zn—O-basedmaterial, a Sn—Zn—O-based material, an Al—Zn—O-based material, anIn—O-based material, a Sn—O-based material, or a Zn—O-based material canbe used. In addition, the material may contain SiO₂.

The oxide semiconductor film 430 can be formed by sputtering in a raregas (typically argon) atmosphere, an oxygen atmosphere, or an atmosphereincluding a rare gas (typically argon) and oxygen.

Here, a 30-nm-thick oxide semiconductor layer is formed by sputteringwith the use of an In—Ga—Zn—O-based metal oxide target that contains In,Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:2 in a molar ratio). Note that asputtering gas has a flow rate of Ar/O₂=0/20 sccm (oxygen: 100%); thetemperature of the substrate is room temperature; deposition pressure is0.6 Pa; and deposition power is 0.5 kW.

Note that the oxide semiconductor film 430 is preferably formed so as tocontain impurities such as hydrogen and water as little as possible.

Then, the oxide semiconductor film 430 is selectively etched through asecond photolithography process, so that an island-shaped oxidesemiconductor layer 431 is formed (see FIG. 9B). The oxide semiconductorfilm 430 can be etched by wet etching. However, this embodiment is notlimited to this. The oxide semiconductor film 430 may be etched by dryetching.

Then, first heat treatment is performed on the oxide semiconductor layer431. Excessive water (including a hydroxyl group), hydrogen, or the likecontained in the oxide semiconductor layer 431 can be removed by thefirst heat treatment. The temperature of the first heat treatment ishigher than or equal to 350° C. and lower than the strain point of thesubstrate, preferably higher than or equal to 400° C. and lower than thestrain point of the substrate.

The oxide semiconductor layer can be dehydrated or dehydrogenated whenthe first heat treatment is performed at a temperature of 350° C. orhigher, so that the concentration of hydrogen in the oxide semiconductorlayer can be lowered. When the first heat treatment is performed at atemperature of 450° C. or higher, the concentration of hydrogen in theoxide semiconductor layer can be further lowered. When the first heattreatment is performed at a temperature of 550° C. or higher, theconcentration of hydrogen in the oxide semiconductor layer can befurther lowered

As the atmosphere in which the first heat treatment is performed, it ispreferable to employ an inert gas that contains nitrogen or a rare gas(e.g., helium, neon, or argon) as a main component and does not containwater, hydrogen, or the like. For example, the purity of a gasintroduced into a heat treatment apparatus can be 6N (99.9999%) or more,preferably 7N (99.99999%) or more. Thus, during the first heattreatment, it is possible to prevent entry of water or hydrogen whilethe oxide semiconductor layer 431 is not exposed to the air.

Note that the heat treatment apparatus is not limited to an electricfurnace, and may be provided with a device for heating an object to beprocessed by thermal conduction or thermal radiation from a heater suchas a resistance heater. For example, an RTA (rapid thermal annealing)apparatus such as a GRTA (gas rapid thermal annealing) apparatus or anLRTA (lamp rapid thermal annealing) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (all electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. AGRTA apparatus is an apparatus with which heat treatment is performedusing a high-temperature gas. An inert gas which does not react with anobject to be processed by heat treatment, nitrogen or a rare gas (e.g.,argon), is used as the gas.

In this embodiment, as the first heat treatment, heat treatment isperformed at 650° C. for six minutes in a nitrogen atmosphere with theuse of a GRTA apparatus.

In addition, the first heat treatment for the oxide semiconductor layercan be performed on the oxide semiconductor film 430 before beingprocessed into the island-shaped oxide semiconductor layer. In thatcase, after the first heat treatment, the second photolithographyprocess is performed.

The first heat treatment for the oxide semiconductor layer may beperformed after a source electrode layer and a drain electrode layer areformed over the oxide semiconductor layer or after a protectiveinsulating film is formed over the source electrode layer and the drainelectrode layer.

After that, a conductive layer is formed so as to cover the gateinsulating layer 402 and the oxide semiconductor layer 431 and is etchedthrough a third photolithography process, so that a source electrodelayer 415 a and a drain electrode layer 415 b are formed (see FIG. 9C).

As the material of the conductive layer, an element selected fromaluminum, chromium, copper, tantalum, titanium, molybdenum, or tungsten;an alloy containing the element as a component; or the like can be used.A material including aluminum and one or more elements selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used. A material selected from manganese, magnesium,zirconium, beryllium, or yttrium may be used. Alternatively, a materialincluding aluminum and one or more elements selected from titanium,tantalum, tungsten, molybdenum, chromium, neodymium, or scandium may beused.

Alternatively, the conductive layer may be formed using an oxideconductive film. As the oxide conductive film, indium oxide (In₂O₃), tinoxide (SnO₂), zinc oxide (ZnO), an alloy of indium oxide and tin oxide(In₂O₃—SnO₂, which is abbreviated as ITO in some cases), an alloy ofindium oxide and zinc oxide (In₂O₃—ZnO), or any of these oxideconductive materials including silicon or silicon oxide can be used.

In that case, a material whose conductivity is higher or whoseresistivity is lower than a material used for the oxide semiconductorlayer 431 is preferably used as the material of the oxide conductivefilm. The conductivity of the oxide conductive film can be increased bythe increase in carrier concentration. Further, the carrierconcentration in the oxide conductive film can be increased by theincrease in hydrogen concentration or the increase in oxygen deficiency.

The source electrode layer 415 a and the drain electrode layer 415 b mayhave a single-layer structure or a layered structure including two ormore layers.

In this embodiment, a 100-nm-thick first titanium layer, a 200-nm-thickaluminum layer, and a 100-nm-thick second titanium layer aresequentially formed over the oxide semiconductor layer 431. Then, astack film including the first titanium layer, the aluminum layer, andthe second titanium layer is etched, so that the source electrode layer415 a and the drain electrode layer 415 b are formed (see FIG. 9C).

In the case where heat treatment is performed after the formation of theconductive layer, a conductive layer which has heat resistance highenough to withstand the heat treatment is used.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor layer 431 is not removedwhen the conductive layer is etched.

Note that through the third photolithography process, only part of theoxide semiconductor layer 431 is etched, so that an oxide semiconductorlayer having a groove (a depression) is formed in some cases.

In order to reduce the number of photomasks used in the photolithographyprocesses and the number of processes, an etching process may beperformed using a multi-tone mask which is an exposure mask throughwhich light is transmitted to have a plurality of intensities. A resistmask formed using a multi-tone mask has a plurality of thicknesses andcan be changed in shape by ashing; thus, the resist mask can be used ina plurality of etching processes for processing films into differentpatterns. Therefore, a resist mask corresponding to at least two or morekinds of different patterns can be formed by one multi-tone mask. Thus,the number of exposure masks and the number of correspondingphotolithography processes can be reduced, so that the process can besimplified.

Next, plasma treatment is performed using a gas such as nitrous oxide(N₂O), nitrogen (N₂), or argon (Ar). With this plasma treatment,absorbed water and the like which attach to a surface of the oxidesemiconductor layer exposed are removed. Alternatively, plasma treatmentmay be performed using a mixture gas of oxygen and argon.

After the plasma treatment, an oxide insulating layer 416 which servesas a protective insulating film and is in contact with part of the oxidesemiconductor layer is formed without exposure to the air (see FIG. 9D).

The oxide insulating layer 416 can be formed by a method by which animpurity such as water or hydrogen is not mixed, such as sputtering. Thethickness of the oxide insulating layer 416 can be at least 1 nm ormore. When hydrogen is contained in the oxide insulating layer 416,hydrogen enters the oxide semiconductor layer 431, so that a backchannelof the oxide semiconductor layer 431 has lower resistance (has n-typeconductivity) and a parasitic channel might be formed. Therefore, it isimportant that a deposition method in which hydrogen is not used beemployed in order that the oxide insulating layer 416 contain as littlehydrogen as possible.

The substrate temperature at the time of deposition is in the range offrom room temperature to 300° C. Further, a deposition atmosphere can bea rare gas (typically argon) atmosphere, an oxygen atmosphere, or anatmosphere including a rare gas (typically argon) and oxygen.

In this embodiment, the substrate is heated at a temperature of 200° C.before the formation of the oxide insulating layer 416, and a300-nm-thick silicon oxide film is formed as the oxide insulating layer416 so as to cover the source electrode layer 415 a and the drainelectrode layer 415 b. The silicon oxide film is formed using a silicontarget by sputtering in which oxygen is used as a sputtering gas.

Then, second heat treatment (preferably at 200 to 400° C., for example,250 to 350° C.) is performed in an inert gas atmosphere or an oxygen gasatmosphere. For example, the second heat treatment is performed at 250°C. for one hour in a nitrogen atmosphere. Through the second heattreatment, part of the oxide semiconductor layer (a channel formationregion) is heated while being in contact with the oxide insulating layer416. By the second heat treatment, oxygen can be supplied to the part ofthe oxide semiconductor layer (the channel formation region). Thus, achannel formation region 413 which overlaps with the gate electrodelayer 411 can be intrinsic. A source region 414 a which overlaps withthe source electrode layer 415 a and a drain region 414 b which overlapswith the drain electrode layer 415 b are formed in a self-aligningmanner. Through the steps, the transistor 410 is formed.

A protective insulating layer may be formed over the oxide insulatinglayer 416. For example, a silicon nitride film can be formed by RFsputtering. Since RF sputtering has high productivity, it is preferablyused as a deposition method of the protective insulating layer. Theprotective insulating layer is preferably formed using an inorganicinsulating film which does not contain an impurity such as moisture, ahydrogen ion, and OH⁻ and blocks entry of such an impurity from theoutside. In this embodiment, as the protective insulating layer, aprotective insulating layer 403 is formed using a silicon nitride film(see FIG. 9D).

Further, heat treatment may be performed at 100 to 200° C. for 1 to 30hours in an air atmosphere. Here, the heat treatment is performed at150° C. for 10 hours. This heat treatment may be performed at a fixedheating temperature. Alternatively, the following change in the heatingtemperature may be conducted plural times repeatedly: the heatingtemperature is increased from room temperature to a certain temperatureof 100 to 200° C. and then decreased to room temperature. Further, thisheat treatment may be performed under a reduced pressure before theformation of the oxide insulating layer. When the heat treatment isperformed under a reduced pressure, the heating time can be shortened.Through this heat treatment, hydrogen is introduced from the oxidesemiconductor layer 431 to the oxide insulating layer 416. That is,hydrogen can be further removed from the oxide semiconductor layer.

A bias temperature test (BT test) is performed on the transistor 410 at85° C. and 2×10⁶ V/cm for 12 hours. As a result, the electricalcharacteristics of the transistor hardly changed and a transistor withstable electrical characteristics was able to be obtained.

The transistor including an oxide semiconductor in this embodiment hasan electrical characteristic of much lower off-state current than atransistor including silicon or the like. Therefore, when the transistorincluding an oxide semiconductor in this embodiment is used as atransistor in a pixel portion, an image signal written to a displayelement can be held for a long period without a change in the circuitstructure or the like of the pixel. Accordingly, in the case where stillimages or the like is displayed, write frequency can be lowered. Thus,power consumption can be reduced.

With the transistor, an image signal written to the display element canbe held for a long period. Accordingly, even in the case where writefrequency is low, degradation (change) in display of the pixel can besuppressed.

A substrate including the transistor in this embodiment can be used asthe substrate 301 illustrated in FIG. 1, FIGS. 2A to 2E, FIGS. 3A and3B, FIG. 4, and FIG. 6. That is, the substrate 601 illustrated in FIGS.2A to 2E and FIGS. 3A and 3B can be attached to the substrate includingthe transistor in this embodiment.

This embodiment can be combined with any of the other embodiments andthe examples as appropriate.

Example 1

In this example, the evaluation of a transistor in a pixel portionincluded in a semiconductor device which is one embodiment of thedisclosed invention is described with reference to FIG. 11, FIGS. 12Aand 12B, and FIGS. 13A and 13B. In this example, measured values ofoff-state current in a test element group (also referred to as TEG) aredescribed below.

FIG. 11 illustrates the initial characteristics of a transistor withL/W=3 μm/10000 μm in which 200 transistors with L/W=3 μm/50 μm each areconnected in parallel. The transistor includes a high-purity oxidesemiconductor layer in a channel formation region. In addition, the topview of the transistor is illustrated in FIG. 12A and a partly enlargedtop view thereof is illustrated in FIG. 12B. The region enclosed by adotted line in FIG. 12B is a transistor of one stage with L/W=3 μm/50 μmand L_(ov)=1.5 μm. Note that here, L_(ov) represents the length of aregion where a source electrode layer or a drain electrode layeroverlaps with an oxide semiconductor layer in a channel lengthdirection. In order to measure the initial characteristics of thetransistor, a change in characteristics of source-drain current(hereinafter referred to as drain current or I_(d)) when source-gatevoltage (hereinafter referred to as gate voltage or V_(g)) is changed,i.e., V_(g)−I_(d) characteristics were measured under condition that thesubstrate temperature was room temperature, source-drain voltage(hereinafter referred to as drain voltage or V_(d)) was 1 V or 10 V, andV_(g) was changed from −20 to +20 V. Note that FIG. 11 illustrates V_(g)in the range of from −20 to +5 V.

As illustrated in FIG. 11, the transistor with a channel width W of10000 μm has an off-state current of 1×10⁻¹³ A or less at V_(d) of 1 Vand 10 V, which is less than or equal to the detection limit of ameasurement device (a semiconductor parameter analyzer, Agilent 4156Cmanufactured by Agilent Technologies Inc.). That is, it is confirmedthat the off-state current of the transistor per micrometer in channelwidth is 10 aA/μm or less. Note that in the case where the channellength is 3 μm or more, the estimated off-state current of thetransistor per micrometer in channel width is 10 aA/μm or less.

Further, a transistor whose channel width W is 1000000 μm (1 m) wasformed similarly and measurement was conducted. As a result, it wasconfirmed that the off-state current is 1×10⁻¹² A or less, which isclose to the detection limit of the measurement device. That is, it wasconfirmed that the off-state current of the transistor per micrometer inchannel width is 1 aA/μm or less.

A method for manufacturing the transistor used for the measurement isdescribed.

First, as a base layer, by CVD, a silicon nitride layer was formed overa glass substrate and a silicon oxynitride layer was formed over thesilicon nitride layer. Over the silicon oxynitride layer, a tungstenlayer was formed as a gate electrode layer by sputtering. Here, thetungsten layer was selectively etched so that the gate electrode layerwas formed.

Next, over the gate electrode layer, a 100-nm-thick silicon oxynitridelayer was formed as a gate insulating layer by CVD.

Then, a 50-nm-thick oxide semiconductor layer was formed over the gateinsulating layer by sputtering with the use of an In—Ga—Zn—O-based metaloxide target (In₂O₃: Ga₂O₃: ZnO=1:1:2 in a molar ratio). After that, anisland-shaped oxide semiconductor layer was formed by selective etchingof the oxide semiconductor layer.

Then, first heat treatment was performed on the oxide semiconductorlayer in a clean oven at 450° C. for one hour in a nitrogen atmosphere.

Then, as a source electrode layer and a drain electrode layer, a150-nm-thick titanium layer was formed over the oxide semiconductorlayer by sputtering. Here, the source electrode layer and the drainelectrode layer were selectively etched, and 200 transistors each havinga channel length L of 3 μm and a channel width W of 50 μm were connectedin parallel so that a transistor with L/W=3 μm/10000 μm is obtained.

Then, as a protective insulating layer, a 300-nm-thick silicon oxidelayer was formed so as to be in contact with the oxide semiconductorlayer by reactive sputtering. Here, the silicon oxide which is aprotective layer was selectively etched so that openings were formedover the gate electrode layer, the source electrode layer, and the drainelectrode layer. After that, second heat treatment was performed at 250°C. for one hour in a nitrogen atmosphere.

Then, heat treatment was performed at 150° C. for 10 hours before themeasurement of V_(g)−I_(d) characteristics.

Through the steps, a bottom-gate transistor was manufactured.

The reason why the off-state current of the transistor is approximately1×10⁻¹³ A as illustrated in FIG. 11 is that the concentration ofhydrogen in the oxide semiconductor layer can be sufficiently reduced inthe manufacturing steps.

The carrier concentration in the oxide semiconductor layer that ismeasured by a carrier measurement device is lower than 1×10¹²/cm³,preferably lower than 1×10¹¹/cm³. That is, the carrier concentration inthe oxide semiconductor layer can be extremely close to zero.

Further, the channel length L of the transistor can be 10 to 1000 nm.Thus, a circuit can operate at higher speed. Furthermore, since theamount of off-state current is extremely small, power consumption can bereduced.

In circuit design, the oxide semiconductor layer can be regarded as aninsulator when the transistor is off.

After that, the temperature characteristics of off-state current of thetransistor manufactured in this example were evaluated. The temperaturecharacteristics are important in considering the environmentalresistance, maintenance of performance, or the like of an end product inwhich the transistor is used. It is to be understood that a smalleramount of change is preferable, which increases the degree of freedomfor product design.

For the temperature characteristics, the V_(g)−I_(d) characteristicswere obtained using a constant-temperature chamber under conditions thatsubstrates provided with transistors were kept at constant temperaturesof −30° C., 0° C., 25° C., 40° C., 60° C., 80° C., 100° C., and 120° C.,drain voltage was 6 V, and gate voltage was changed from −20 to +20 V.

FIG. 13A illustrates V_(g)−I_(d) characteristics measured at thetemperatures and superimposed on one another, and FIG. 13B illustratesan enlarged view of the range of off-state current enclosed by a dottedline in FIG. 13A. The rightmost curve indicated by an arrow in thediagram is a curve obtained at −30° C.; the leftmost curve is a curveobtained at 120° C.; and curves obtained at the other temperatures arelocated therebetween. The temperature dependence of on-state current canhardly be observed. On the other hand, as clearly illustrated also inthe enlarged view of FIG. 13B, the off-state current is 1×10⁻¹² A orless, which is near the detection limit of the measurement device, atall the temperatures except the case where the gate voltage is around−20 V, and the temperature dependence thereof is not observed. In otherwords, even at a high temperature of 120° C., the off-state current iskept at 1×10⁻¹² A or less, and given that the channel width W is 10000μm, it can be seen that the off-state current is significantly low. Thatis, it is confirmed that the off-state current of the transistor permicrometer in channel width is 100 aA/μm or less. Note that in the casewhere the channel length is 3 μm or more, the estimated off-statecurrent of the transistor per micrometer in channel width is 100 aA/μmor less.

As described above, a transistor including a high-purity oxidesemiconductor shows almost no dependence of off-state current ontemperature. It can be said that an oxide semiconductor does not showtemperature dependence when highly purified because the conductivitytype becomes extremely close to an intrinsic type and the Fermi level islocated in the middle of the forbidden band. This also results from thefact that the oxide semiconductor has a large energy gap and includesvery few thermally excited carriers.

The results show that the off-state current of a transistor whosecarrier density is lower than 1×10¹²/cm³, preferably lower than1×10¹¹/cm³ at room temperature is 1 aA/μm or lower. In addition, whenthe transistor is used as a transistor included in a semiconductordevice, the power consumption of the semiconductor device can be reducedand degradation in display (the decrease in display quality) can besuppressed. Further, it is possible to provide a semiconductor devicewhere degradation (change) in display due to an external factor such astemperature is suppressed.

Therefore, when a transistor including a high-purity oxide semiconductoris used as a transistor in a pixel portion as described above, an imagesignal written to a display element can be held for a long periodwithout a change in the circuit structure or the like of the pixel.Accordingly, in the case where still images or the like is displayed,write frequency can be lowered. Thus, power consumption can be reduced.

When the high-purity transistor is used as described above, an imagesignal written to the display element can be held for a long period.Accordingly, even in the case where write frequency is low, degradation(change) in display of the pixel can be suppressed.

As described above, a substrate including the high-purity transistor canbe used as the substrate 301 illustrated in FIG. 1, FIGS. 2A to 2E,FIGS. 3A and 3B, FIG. 4, and FIG. 6. That is, as described above, thesubstrate 601 illustrated in FIGS. 2A to 2E and FIGS. 3A and 3B can beattached to the substrate including the high-purity transistor.

Example 2

In this example, the evaluation of a transistor in a pixel portionincluded in a semiconductor device which is one embodiment of thedisclosed invention is described with reference to FIG. 14, FIG. 15, andFIG. 16. In this example, the off-state current of a transistorincluding a high-purity oxide semiconductor is accurately obtained, andresults thereof are shown.

First, a test element group used in a method for measuring current isdescribed with reference to FIG. 14. In the test element group in FIG.14, three measurement systems 800 are connected in parallel. Themeasurement system 800 includes a capacitor 802, a transistor 804, atransistor 805, a transistor 806, and a transistor 808. The transistor804, the transistor 805, and the transistor 806 are formed in accordancewith the manufacturing method illustrated in FIGS. 9A to 9D and have astructure which is similar to that of FIG. 9D.

In the measurement system 800, one of a source terminal and a drainterminal of the transistor 804, one terminal of the capacitor 802, andone of a source terminal and a drain terminal of the transistor 805 areconnected to a power source (a power source for supplying V2). The otherof the source terminal and the drain terminal of the transistor 804, oneof a source terminal and a drain terminal of the transistor 808, theother terminal of the capacitor 802, and a gate terminal of thetransistor 805 are connected to each other. The other of the sourceterminal and the drain terminal of the transistor 808, one of a sourceterminal and a drain terminal of the transistor 806, and a gate terminalof the transistor 806 are connected to a power source (a power sourcefor supplying V1). The other of the source terminal and the drainterminal of the transistor 805 and the other of the source terminal andthe drain terminal of the transistor 806 are connected to each other andserve as an output terminal Vout.

A potential V_(ext) _(—) _(b2) for controlling on/off of the transistor804 is supplied to a gate terminal of the transistor 804. A potentialV_(ext) _(—) _(b1) for controlling on/off of the transistor 808 issupplied to a gate terminal of the transistor 808. Further, a potentialV_(out) is output from the output terminal.

Next, a method for measuring off-state current with the measurementsystem is described.

First, an initial period in which a potential difference is generated inorder to measure off-state current is described. In the initial period,the potential V_(ext) _(—) _(b1) for turning on the transistor 808 isinput to the gate terminal of the transistor 808, and the potential V1is supplied to a node A connected to the other of the source terminaland the drain terminal of the transistor 804 (i.e., a node connected tothe one of the source terminal and the drain terminal of the transistor808, the other terminal of the capacitor 802, and a gate terminal of thetransistor 805). Here, the potential V1 is, for example, a highpotential. Further, the transistor 804 is off.

After that, the potential V_(ext) _(—) _(b1) for turning off thetransistor 808 is input to the gate terminal of the transistor 808 sothat the transistor 808 is turned off. After the transistor 808 isturned off, the potential V1 is set low. The transistor 804 is kept off.Further, the potential V2 is set low. Thus, the initial period isfinished.

Next, a measurement period of the off-state current is described. In themeasurement period, the potential (i.e., V2) of the one of the sourceterminal and the drain terminal of the transistor 804 and the potential(i.e., V1) of the other of the source terminal and the drain terminal ofthe transistor 808 are fixed low. On the other hand, the potential ofthe node A is not fixed (the node A is in a floating state) in themeasurement period. Accordingly, electrical charges flow through thetransistor 804, and the amount of electrical charges stored in the nodeA is changed as time passes. The potential of the node A is changeddepending on the change in the amount of electrical charges stored inthe node A. That is, the output potential Vout of the output terminal isalso changed. The off-state current can be calculated from the thusobtained output potential Vout.

Each of the transistor 804, the transistor 805, the transistor 806, andthe transistor 808 is a transistor including a high-purity oxidesemiconductor with a channel length L of 10 μm and a channel width W of50 μm. In the three measurement systems 800 connected in parallel, thecapacitance value of the capacitor 802 in a first measurement system is100 fF; the capacitance value of the capacitor 802 in a secondmeasurement system is 1 pF; and the capacitance value of the capacitor802 in a third measurement system is 3 pF.

Note that V_(DD) was 5 V and V_(SS) was 0 V in the measurement ofoff-state current. In the measurement period, the potential V1 wasbasically V_(SS) and V_(DD) only in a period of 100 msec every 10 to 300seconds, and V_(out) was measured. Further, time taken to calculatecurrent I flowing through an element was about 30000 seconds.

FIG. 15 shows the relationship between elapsed time Time in measurementof the current and the output potential V_(out). FIG. 15 shows that thepotential varies as time passes.

FIG. 16 shows off-state current calculated in the measurement of thecurrent. Note that FIG. 16 shows the relationship between source-drainvoltage V and off-state current I. FIG. 16 shows that off-state currentis about 40 zA/μm under the condition that the source-drain voltage is 4V. In addition, the off-state current is less than or equal to 10 zA/μmunder the condition that the source-drain voltage is 3.1 V. Note that 1zA represents 10⁻²¹ A.

According to this example, it is confirmed that the off-state currentcan be sufficiently low in a transistor including a high-purity oxidesemiconductor.

When a transistor including a high-purity oxide semiconductor is used asa transistor in a pixel portion as described above, an image signalwritten to a display element can be held for a long period without achange in the circuit structure or the like of the pixel. Accordingly,in the case where still images or the like is displayed, write frequencycan be lowered. Thus, power consumption can be reduced.

When the high-purity transistor is used as described above, an imagesignal written to the display element can be held for a long period.Accordingly, even in the case where write frequency is low, degradation(change) in display of the pixel can be suppressed.

As described above, a substrate including the high-purity transistor canbe used as the substrate 301 illustrated in FIG. 1, FIGS. 2A to 2E,FIGS. 3A and 3B, FIG. 4, and FIG. 6. That is, as described above, thesubstrate 601 illustrated in FIGS. 2A to 2E and FIGS. 3A and 3B can beattached to the substrate including the high-purity transistor.

This application is based on Japanese Patent Application serial No.2010-019602 filed with Japan Patent Office on Jan. 29, 2010, the entirecontents of which are hereby incorporated by reference.

1. A semiconductor device comprising: a first substrate; a secondsubstrate; a first signal antenna provided over a surface side of thefirst substrate; and a second signal antenna and an integrated circuitprovided over the second substrate, wherein the second substrate isattached to a back side of the first substrate, and wherein the firstsignal antenna and the second signal antenna overlap with each otherwith the first substrate provided therebetween.
 2. The semiconductordevice according to claim 1, each of the first signal antenna and thesecond signal antenna serves as a power antenna.
 3. The semiconductordevice according to claim 1, further comprising: a first power antennaprovided over the surface side of the first substrate; and a secondpower antenna provided over the second substrate, wherein the firstpower antenna and the second power antenna overlap with each other withthe first substrate provided therebetween.
 4. The semiconductor deviceaccording to claim 1, wherein each of the first substrate and the secondsubstrate is a flexible substrate.
 5. The semiconductor device accordingto claim 1, further comprising a pixel portion including a plurality ofpixels over the first substrate, wherein each of the plurality of pixelsincludes a transistor and a display element.
 6. The semiconductor deviceaccording to claim 5, wherein a channel formation region of thetransistor includes an oxide semiconductor layer.
 7. The semiconductordevice according to claim 1, further comprising a scan line drivercircuit, a signal line driver circuit, and a pixel portion including aplurality of pixels over the first substrate, wherein each of theplurality of pixels includes a transistor whose on/off is controlled bythe scan line driver circuit, and includes a display element to which animage signal is input from the signal line driver circuit through thetransistor.
 8. The semiconductor device according to claim 7, wherein achannel formation region of the transistor includes an oxidesemiconductor layer.
 9. A semiconductor device comprising: a firstsubstrate; a second substrate; a first signal antenna provided over asurface side of the first substrate; and a second signal antenna and anintegrated circuit provided over the second substrate, wherein thesecond substrate is attached to a back side of the first substrate withan adhesive, and wherein the first signal antenna and the second signalantenna overlap with each other with the first substrate providedtherebetween.
 10. The semiconductor device according to claim 9, each ofthe first signal antenna and the second signal antenna serves as a powerantenna.
 11. The semiconductor device according to claim 9, furthercomprising: a first power antenna provided over the surface side of thefirst substrate; and a second power antenna provided over the secondsubstrate, wherein the first power antenna and the second power antennaoverlap with each other with the first substrate provided therebetween.12. The semiconductor device according to claim 9, wherein each of thefirst substrate and the second substrate is a flexible substrate. 13.The semiconductor device according to claim 9, further comprising apixel portion including a plurality of pixels over the first substrate,wherein each of the plurality of pixels includes a transistor and adisplay element.
 14. The semiconductor device according to claim 13,wherein a channel formation region of the transistor includes an oxidesemiconductor layer.
 15. The semiconductor device according to claim 9,further comprising a scan line driver circuit, a signal line drivercircuit, and a pixel portion including a plurality of pixels over thefirst substrate, wherein each of the plurality of pixels includes atransistor whose on/off is controlled by the scan line driver circuit,and includes a display element to which an image signal is input fromthe signal line driver circuit through the transistor.
 16. Thesemiconductor device according to claim 15, wherein a channel formationregion of the transistor includes an oxide semiconductor layer.
 17. Asemiconductor device comprising: a first substrate; a second substrate;a first signal antenna provided over a surface side of the firstsubstrate; and a second signal antenna and an integrated circuitprovided over the second substrate, wherein the second substrate isattached to a back side of the first substrate with an adhesive, whereinan insulating filler is mixed into the adhesive, and wherein the firstsignal antenna and the second signal antenna overlap with each otherwith the first substrate provided therebetween.
 18. The semiconductordevice according to claim 17, each of the first signal antenna and thesecond signal antenna serves as a power antenna.
 19. The semiconductordevice according to claim 17, further comprising: a first power antennaprovided over the surface side of the first substrate; and a secondpower antenna provided over the second substrate, wherein the firstpower antenna and the second power antenna overlap with each other withthe first substrate provided therebetween.
 20. The semiconductor deviceaccording to claim 17, wherein each of the first substrate and thesecond substrate is a flexible substrate.
 21. The semiconductor deviceaccording to claim 17, further comprising a pixel portion including aplurality of pixels over the first substrate, wherein each of theplurality of pixels includes a transistor and a display element.
 22. Thesemiconductor device according to claim 21, wherein a channel formationregion of the transistor includes an oxide semiconductor layer.
 23. Thesemiconductor device according to claim 17, further comprising a scanline driver circuit, a signal line driver circuit, and a pixel portionincluding a plurality of pixels over the first substrate, wherein eachof the plurality of pixels includes a transistor whose on/off iscontrolled by the scan line driver circuit, and includes a displayelement to which an image signal is input from the signal line drivercircuit through the transistor.
 24. The semiconductor device accordingto claim 23, wherein a channel formation region of the transistorincludes an oxide semiconductor layer.